Wifey set off to her evening job in M&S only to return some minutes later saying "I've got a flat battery".
I ran off to the garage to fetch a spare car battery and some jump leads.
When I got the car I noticed that the interior light came on when I opened the door. Sure enough the battery wasn't flat, but the car was refusing to fire.
Hmmmm.
Wifey called out the RAC and almost immediately after arriving the bloke asked her "Have you filled up recently"
"Yes, last night just before I got home" she replied.
"Can I see the receipt?" he asked.
You guessed it, she'd filled up with diesel.
I did laugh, but only out of earshot :-)
GC